Summary of work: Several cellular markers of oxidative stress are higher in cells from Alzheimer disease (AD) patients as compared to normal age-matched controls. These markers include oxidative damage to lipids, proteins and DNA in various tissues from AD subjects. It has been proposed that AD cells may have a defect in the DNA repair processing of oxidative base lesion leading to accumulation of DNA damage. Using DNA substrates containing both pyrimidine and purine lesions we have investigated the repair of oxidative base lesions in whole cell extracts from cultured AD lymphoblasts. Our data indicate that oxidative DNA lesions are repaired efficiently in AD lymphoblasts compared to controls. In normal cells, oxidative DNA damage is mainly repaired by the base excision repair (BER) pathway. We are investigating the hypothesis that DNA repair is modulated in AD by measuring BER capacity in whole cell and mitochondrial extracts obtained from well established animal models for AD, since the mitochondrial DNA seems to be preferentially target for oxidative damage. Recent reports have shown that corpus callosum and hippocampus atrophy are hallmarks of AD brains, suggesting that some regions are more susceptible to AD-induced degeneration. Thus, we are measuring repair capacity in extracts of different brain regions in normal and AD-model mice. We also follow age-associated changes in DNA repair capacity in these regions. Our results show that BER activities in mitochondria varied greatly among 5 brain regions, striatum, frontal cortex, cerebellum, hippocampus and brain steam, with brain steam having highest and striatum the lowest DNA glycosylase activities. We observed a general decrease in BER efficiency with age; however the age-associated changes also differ among the regions. Our results suggest that differential changes of BER activity with age may play a role in the etiology of various neurodegenerative processes. To better understand the role of oxidative damage to mtDNA in AD we have used a mouse model system for AD, transgenic mice expressing the amyloid precursor protein 1 (APP1) gene. APP1 is the precursor of the beta amyloid protein, which accumulates in the brain of AD patients and has been shown to induce 8OHdG accumulation in mtDNA. Our results show that liver mitochondria from APP1 mice have slightly higher repair of 8OHdG than wt animals. However, mitochondria obtained from APP1 mice kept in a folic acid deficient diet show significantly higher 8OHdG incision activity that wt. Since the folic acid deficient diet is associated with higher levels of homocysteine and oxidative stress, these results are consistent with an up-regulation of BER. We are now directly testing the hypothesis that 8OHdG accumulation plays a role in neurodegenerative processes. For that, mice deficient in the oxoguanine DNA glycosylase (OGG1) are injected with model drugs for neurodegenerative diseases, such as MPTP and kainic acid. These animals completely lack 8OHdG removal in mitochondria and show decrease activity in the nuclei. The results obtained from these studies will help advance our understanding on the molecular events that play direct causal roles in the pathology of AD